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The sharpest image ever taken of a dust disk around another star has revealed structures in the disk which are signs of unseen planets.

Dust grains orbiting the star AU Mic. The light from the star itself has been been removed from the center of this image, which was obtained with the Keck Telescope on Mauna Kea. Irregularities in the shape of the disk indicate the presence of unseen planets in orbit around the star. The image is 100 Astronomical Units wide (9.3 billion miles), about the size of our solar system. Image Credit: M. Liu, IfA-Hawaii/Keck Observatory

Dr. Michael Liu, an astronomer at the University of Hawaii's Institute for
Astronomy, has acquired high resolution images of the nearby star AU
Microscopii (AU Mic) using the Keck II Telescope, the world's largest
infrared telescope. At a distance of only 33 light years, AU Mic is the
nearest star possessing a visible disk of dust. Such disks are believed to
be the birthplaces of planets.

"We cannot yet directly image young planets around AU Mic, but they cannot
completely hide from us either. They reveal themselves through their
gravitational influence, forming patterns in the sea of dust
grains orbiting the star," said Dr. Liu.

The results are published in the August 12th online Science Express and
in the September print edition of Science.

A dust disk ordinarily would appear relatively featureless and symmetric,
because any disturbances would be smoothed out as the material orbits the
star. However, this is not observed in the case of AU Mic. Instead, Dr.
Liu has found its disk is uneven and possesses clumps. These structures
arise and are maintained due to the gravitational influence of unseen
planetary companions.

The clumps in AU Mic's disk lie at separations of 25 to 40 Astronomical
Units away from the central star (where one Astronomical Unit is the
distance from the Earth to the Sun), or about 2 to 4 billion miles. In
our own solar system, this corresponds to the regions where Neptune and
Pluto reside.

AU Mic is a dim red star, with only half the mass and one-tenth the energy
output as the Sun. Previous studies have shown that AU Mic is about 12
million years old, an epoch believed to be an active phase of
planet formation. In comparison, our Sun is about 4.6 billion years old, or
about 400 times older, and planet formation has long since ended.

"By studying very young stars like AU Mic, we gain insight into the planet
formation process as it is occurring. As a result, we learn about the birth
of our own solar system and its planets," said Liu.

The images alone cannot yet tell us what kinds of planets are present, only
that the planets are massive enough to gravitationally alter the
distribution of the dust. However, many structures in the AU Mic disk
are observed to be elliptical (non-circular), indicating that the planetary
orbits are elliptical. This is different than in our own solar system,
where most planets follow circular orbits.

Images of disks around nearby stars are very rare. Earlier this year, Dr.
Liu and his colleagues announced the discovery of the large dusty disk
around AU Mic. The light from AU Mic's disk arises from small dust
particles which reflect the light of the central star. The new images are
30 times sharper than the earlier ones, enabling discovery of the clumps in
the inner disk of AU Mic.

Dr. Liu's used the Keck II Telescope located on Mauna Kea, Hawaii for this
research. The two Keck Telescopes are the largest infrared telescopes in
the world, each with a primary mirror of 10-meter (33 feet) in diameter.
The telescopes are equipped with adaptive optics, a powerful technology
which corrects astronomical images for the blurring caused by the Earth's
turbulent atmosphere.

The resulting infrared images are the sharpest ever obtained of a
circumstellar disk, with an angular resolution of 1/25 of an arcsecond,
about 1/500,000 the diameter of the full moon. If a person's vision
were as sharp as the Keck adaptive optics system, he would be able to read a
magazine that was one mile away. In the case of AU Mic, the Keck images can
see features as small as 0.4 Astronomical Units, less than half the distance
from the Earth to the Sun.

"It is remarkable how quickly Adaptive Optics at Keck has come from being an
exotic demonstration technology to producing scientific results of
unprecedented quality," said Dr. Frederic H. Chaffee, the director of the W.
M. Keck Observatory. "We are entering a new age of high resolution imaging
in astronomy. Dr. Liu's breathtaking images of possible planets in
formation around AU Mic would have been unimaginable from any telescope --
space-based or on Earth -- a few short years ago. This is an exciting time
for us all."

This work is supported in part by the National Science Foundation.

The W.M. Keck Observatory is managed by the California Association for
Research in Astronomy, a non-profit 501 (c) (3) corporation. CARA is a
scientific partnership of the California Institute of Technology, the
University of California, and the National Aeronautics and Space
Administration.

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